The purpose of this study is to learn more about how the body controls blood flow in humans. Nitric Oxide (NO) is a vasodilating substance released by endothelial cells which contributes importantly to the regulation of vascular tone. NO also inhibits the adhesion of macrophages and platelets to endothelial cells, one of the early events in the development of atherosclerosis. Recent work indicates a gender- related difference in the amount of NO released from endothelium and that this differnce might be due to the presence of estrogen. In this proposal, the investigators will study the role that gender and exogenous estrogen components have on NO-mediated vasodilation. Information from these studies will lend significant understanding to gender-related differences in the development of vascular disease and have the potential of altering risk in 50% of the population.